Adjective sullen has 2 senses
- dark, dour, glowering, glum, moody, morose, saturnine, sour, sullen - showing a brooding ill humor; "a dark scowl"; "the proverbially dour New England Puritan"; "a glum, hopeless shrug"; "he sat in moody silence"; "a morose and unsociable manner"; "a saturnine, almost misanthropic young genius"- Bruce Bliven; "a sour temper"; "a sullen crowd"
Antonym: good-natured (indirect, via ill-natured)
- heavy, lowering, sullen, threatening - darkened by clouds; "a heavy sky"
Antonym: clear (indirect, via cloudy)
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